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The leg extension machine focuses on the four muscles of the quadriceps. Involving only the knee joint, leg extensions are an isolation movement that develops strength in the quadriceps at the front of the thigh. The exercise forces the quadriceps to extend the leg against resistance, strengthening the muscles in addition to the connective tissue around the knee.

Quadricep Breakdown

Knee extension -- the motion of straightening the leg performed when using the quad extension machine -- is the primary function of the quadriceps. Aptly named the quadriceps on account of the four major sections that make up the muscle, the quadriceps are the most powerful muscle in the body. The four sections of the muscle are the rectus femoris, vastus medialis, vastus intermedalis and vastus lateralis. The quadriceps originate at the hip and insert at the knee cap through a common tendon, according to Human Anatomy and Physiology.

The Perfect Fit

Proper alignment and form when using the leg extension machine ensures you’re targeting the quadriceps effectively without causing damage to the knees or compensating with other muscles to extend the leg. When you are seated on the bench, position the backrest forward enough so that your knees are aligned with the axis of the moving extension bar. If your knees are too far forward of the pivot point, you’ll limit your range of motion and not obtain full extension. If your knees are too far back from the pivot point, you’ll be placing potentially dangerous torque onto the knee joint instead of using the quadriceps to extend the leg. The shins should rest on the lower leg pad, just above the curve where the leg meets the foot. Grasp the side bars and straighten your legs, lifting the bar with a slow and controlled motion. Pause slightly when your legs are straight, and then return to the starting position with a slow and controlled movement.

Change It Up

Despite the isolated nature of the leg extension movement, the exercise can be varied in multiple ways. According to Men’s Health, working one leg at a time helps identify and overcome muscle imbalances. One leg is usually stronger than the other, and working them singly can help even up leg strength. Bodybuilding icon and Mr. Olympia champion Arnold Schwarzenegger states that varying your foot position also varies the section of the quadriceps that is emphasized during exercise. Keeping your feet close together and toes straight ahead places emphasis on the outer thigh, while keeping your feet apart with the toes turned outwards emphasizes the front and inner thigh. You can vary your foot position against the quad extension bar, but use a lighter weight than you would with standard foot positioning. Changing your foot position alters the amount of stress on the knees, so working with lighter weight will protect your knees while still allowing you to target different heads of the quadricep.

Play It Safe

Always start conservatively with weights to allow your knees to adjust to the torque of the exercise. Start with ten minutes of gentle cardio to make sure you are warmed up and to provide a dynamic stretch that will get blood flowing to the correct muscles in your legs and joints. Be sure to use slow and controlled movements throughout the entire range of motion, using the strength of your quadriceps to lift the weight rather than swinging or jerking the leg bar and relying on momentum. Avoid letting the weights slam back onto the stack when returning to the starting position; instead, use your quadriceps to lower the weights with a controlled motion.

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About the Author

Jullie Chung writes regularly for various websites. She is a nationally certified fitness trainer and performance enhancement specialist through the National Academy of Sports Medicine and trains regularly in yoga, flatwater kayaking, boxing and mixed martial arts. An avid outdoor fan, she regularly hikes, climbs and trail runs.